DC series motor

 

DC series motor

DC series motor is one of the commonly used motors for traction purpose. It possesses remarkably good characteristics for traction so it is used heavily in industry and locomotives. DC series motor is simple and robust in nature and suitable for all type of traction purpose but it is specially preferred in the suburban railway system. The various characteristics that made DC series motor useful for traction system are as follows 

 

It has high starting torque which is important in traction system.

1. It has simple speed control methods.
2. It has good commutation property up to twice the full load.
3. It possesses the ability to run in parallel with other loads.
4. It has the capability of withstanding sudden load change.
5. It requires less power for high torque production because torque production is directly 
6. proportional to the square of the armature current.

7. The DC Traction Motor: How it Drives the Axle

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   The traditional DC (Direct Current) electric motor driving a train or locomotive is a simple machine consisting of a case containing a fixed electrical part, the stator (called the stator because it is static and comprising what is called the field coils) and a moving electrical part, the rotor (because it rotates) or armature as it is often called.  As the rotor turns, it turns a pinion which drives a gearwheel.  The gearwheel is shrunk onto the axle and thus drives the wheels as shown in the diagram above.
   
   The motion of the motor is created by the interaction of the magnetism caused by the currents flowing the the stator and the rotor.  This interaction causes the rotor to turn and provide the drive. 
   
   The stator and the rotor of the DC motor are connected electrically.  The connection consists of fixed, carbon brushes which are spring loaded so that they remain in contact with an extension of the armature called the commutator.  In this way, the field coils (the stator) are kept in the circuit with the rotor (the armature and commutator).

    

   AC and DC Motors

   Both AC (Alternating Current) and DC motors have the same basic structure but there are differences and, for various reasons, the DC motor was originally the preferred form of motor for railway applications and most systems used it.  Nowadays, modern power electronics has allowed the use of AC motors and, for most new equipments built today, the AC motor is the type used. 

   Often, people ask about the differences between AC and DC motors as used in locomotives and multiple-units.  In the early days of electric traction at the beginning of this century both types were tried. The limits of the technology at the time favoured the DC motor. It provided the right torque characteristic for railway operation and was reasonably simple to control.

   By the early 1980s, power electronics had progressed to the stage where the 3-phase AC motor became a serious and more efficient alternative to the DC motor because:

   1. They are simpler to construct, they require no mechanical contacts to work (such as brushes) and they are lighter than DC motors for equivalent power.

   2. Modern electronics allow AC motors to be controlled effectively to improve both adhesion and traction.

   3. AC motors can be microprocessor controlled to a fine degree and can regenerate current down to almost a stop whereas DC regeneration fades quickly at low speeds.

   4. They are more robust and easier to maintain than DC motors.

   This type of motor is commonly called the Asynchronous Motor and was often referred to as the squirrel cage motor on account of its early design form.  The photos below show a DC and an AC motor.

   

   

    

   The DC motor is similar to look at externally but there are differences in construction, particularly because the DC motor has a commutator and brushes which the AC motor does not

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   Nose Suspended Motor

   The following diagram shows the layout of the traditional DC motor mounted in a bogie as a "nose suspended motor".

   In electric trains or loco motives, the DC motor was traditionally mounted in the bogie frame supported partially by the axle which it drove and partially by the bogie frame.  The motor case was provided with a "nose" which rested on a bracket fixed to the transom of the bogie.  It was called a "nose suspended motor" (see diagram above) and is still common around the world.  Its main disadvantage is that part of the weight rests on the axle and is therefore unsprung.  This leads to greater wear on bogie and track.  Nowadays, designers try to ensure all the motor weight is sprung by ensuring it is carried entirely by the bogie frame - a frame mounted motor.

   Quill Drive

   This is a simplified diagram of a quill drive.  A quill is described in the dictionary as, "the hollow stem of a feather" and "a bobbin or spindle", as well as a "feather" and, alternatively, what a porcupine has on its back. 
   
   In railway traction terms, a quill drive is where a hollow shaft is placed round the driving axle and the motor drives the quill rather than driving the axle as it does with a nose suspended drive. The quill itself is attached, at one end, to one of the wheels by means of rubber bushed links and, at the other end, to the gearwheel by similar links. The big advantage of such drives is that all the weight of the motor is carried in the bogie frame (so it is a frame mounted motor) instead of it being directly supported by the axle and therefore partially unsprung.

   An example of a traction motor with quill drive appears in the following photo.  Click on it for a full size view and the part names.  Various forms of quill drive have been used over the years.  Older versions used radially mounted coil steel springs instead of rubber to connect the links to the wheels.  Some, like the example shown here, have the motor mounted parallel with the axle.  Others have the motor at a right angle to the axle, as in the the UK Class 91 electric locomotives.
   
   In German the quill is called "Hohlwelle" (hollow shaft) and used in the ICE1 and ICE2 as well as the electric locomotive Class 101. (Source Tobias Benjamin Koehler 19 Oct 98). 

   Monomotor Bogie

   As its name implies, the monomotor bogie has a single motor which drives both axles.  Click on the thumbnail to see a photo with the part names.
   
   The design is much favoured in France, where it was introduced in the 1950s for the rubber tyred train concept.  The motor is mounted longitudinally in the centre of the bogie and drives each axle through a differential gearbox, similar to a road vehicle.  The differential gears are required to compensate for the operation of the rubber tyres round curves.  It requires a special bogie frame construction to accommodate the motor.
   
   Another version of the monomotor bogie has also been applied to a number of French locomotive designs but here the arrangement is more conventional.  Each bogie has a single motor mounted transversely over the centre as shown in the diagram left.

   The motor is fully suspended in the bogie frame and drives both axles through the gear train, which is contained in a single, large, oil filled gearcase (not shown).  This type of drive is referred to in the locomotive wheel arrangement called a B-B, as opposed to a more conventional locomotive with four motors, each driving its own axle, which is called a Bo-Bo.

    

   
   

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